1. Field of the Invention
The invention pertains to a tube fitting for providing a sealed flow connection between an open-ended duct that extends along the longitudinal axis of the tube and an open-ended duct of a base element, for liquid separation technology, in particular for analytical or preparative HPLC, that can be preferably unattached and that comprises a ring-shaped thrust piece which provides a receptacle for the tube, a ring-shaped, elastic compensating element which provides a receptacle for the tube, and a fastening element which preferably is manually operable and which can be tightly fastened to the base element, wherein the tube preferably has a ring flange on its end connected to the base element that protrudes from the longitudinal axis of the tube, and wherein, when sealed, the base element, the thrust piece, the compensating element and the tube can be braced by means of the fastening element under elastic deformation of the compensating element.
2. Discussion of the Background Art
There is a plethora of tube fittings used in a wide variety of applications for connecting tubes with tube connector elements providing flow ducts, for example, fitting connectors, complex hydraulic units, seals or other tubes, such that a sealed flow connection between the tube and the flow duct of the tube connector element results.
Tube fittings in the area of liquid separation technology, used in particular in the area of analytical or preparative HPLC, must fulfill very specific requirements with respect to the materials used on the one hand and the constructive design of the tube fittings on the other hand. Such tube fittings must withstand increases in pressure up to 60 bar and/or temperature increases, and must also enable a permanent sealing of the connectable flow ducts under these conditions as well.
Particularly when implemented in preparative systems, dead zones such as pocket holes or poorly flushed areas must be avoided by all means in order to prevent entrainment of sediment and subsequent band widening.
Tube fittings provided on measuring detectors downstream of separation columns, in particular measuring bulbs, must satisfy the tight spacing conditions caused by the closely adjacent inputs and outputs of the measuring bulbs. This means that the tube fittings must have a low center distance and must be simply and easily manually detachable and reattachable as well. Because the bulbs consist primarily of quartz glass, high-gloss sealing surfaces can be used that enable a secure seal even with relatively low sealing force.
In tube fittings for valves, in particular for fraction collectors such as two/three-way-valves, the base bodies of the valves are made mostly of stainless steel. Due to the usually poor accessibility of sealing surfaces found within the area of pocket-hole type borings, these usually have a relatively rough surface, such that increased sealing forces are required for an adequately secure seal. It is important in this application as well to be able to place the tube fittings within a low center distance in order to avoid undesirable band widening caused by comparatively large valve volumes. Furthermore, these tube fittings must be simply and easily attachable and detachable.
In the area of liquid separation technology there are essentially two known types of purely mechanical, reattachable tube fittings.
In a first type of tube fittings, mostly polymer tubes with smooth edges are used that have an interface normally along the longitudinal axis of the tube. These tube fittings, also known as xe2x80x9cferrulesxe2x80x9d, may either be pre-assembled or not pre-assembled. In pre-assembled tube fittings, a male-threaded clamping screw providing a through-hole, a clamp ring made from a relatively hard material, and finally a polymer seal ring is fitted over the edge which has an inner diameter corresponding to the outer diameter of the tube or which is slightly smaller than the outer diameter of the tube. The clamp ring and the seal ring are coordinated to each other with wedge-shaped peripheral surfaces perpendicular to the grain such that when the clamp ring is fitted over the seal ring, the latter is elastically deformed radially outward such that a clamping force is induced radially between the seal ring and the outer surface of the tube, which forms the seal to the tube. In addition, another axial force component is induced by the clamp ring which, working together with a ring flange of the seal ring, provides a seal between its axial fore-part and the base body or tube connection element respectively. The actual seal to the tube on the one hand and to the base body on the other hand is formed by tightening the clamp screw, whereby its clamp surface, which normally runs along the longitudinal axis of the tube, is pushed in the direction of the parallel fore-part of the clamp ring.
The disadvantage of this simple and cost-efficient tube fitting is that the polymer seal ring creeps over time subject to the active tightening forces and therefore plastically deforms, resulting in respectively diminishing sealing forces and finally to an unsealed connection.
In a second type of tube fittings, used in the area of liquid separation technology, the polymer tubes, which are likewise favorable, have a ring flange on their ends in the shape of a raised-edge, the side parts of which protrude radially outward over the outer diameter of the tube. Either before the creation of the flange or from the opposing end of the tube, a planed perforated plate, a polymer O-ring seal and a male-threaded clamp screw providing a through-hole is fitted over the tube such that the perforated plate lies directly opposite the ring flange of the tube, while the O-ring and finally the clamp screw is arranged in the direction of the end of the tube opposite the ring flange. A clamp screw, which normally has its surface arranged to the longitudinal axis of the tube and the O-ring, is screwed and tightened in a female threading of an opening of the base body or tube connection piece respectively. The front side of the clamp surface of the clamp screw thereby presses against the elastically deformable seal ring. This is supported on the perforated plate, which in turn supports itself off the opposing ring flange of the tube. This can result in undefined rotating and a tumbling motion of the perforated plate when the clamp screw is tightened, resulting in undefined seal conditions at the junction to the ring flange of the tube. Moreover, the ring flange or parts thereof may move off-center or shift relative to the opposing sealing surfaces of the base body, resulting in the formation of undesirable dead spaces. Finally, in this type of tube fitting it is unavoidable that the polymer O-ring creeps over time under the tightening momentum, i.e. plastically deforms over time, resulting in leaks.
Accordingly, an object of the invention is to provide a tube connection that is easy to manage and that can be activated with a low amount of exertion, with which a secure flow connection between the tube and the duct piece of a base element connected with the tube is guaranteed over long periods of time.
This object is carried out according to the invention by means of the features of claim 1, in particular in that a spring which serves as a compensating element is provided with a pre-determined spring excursion, wherein the spring and connection parts within the spring excursion range are coordinated to each other such that a leak in the connection caused by plastic deformation or material settling of parts braced together is excluded with certainty.
The spring is best provided with at least one perforated plate which is arranged preferably either to a middle section or as a disc spring. These seemingly easy steps have surprisingly turned out that contrary to known state of the art tube connections, a sealed connection between flow ducts can be achieved over long periods of time as well.
It is sensible furthermore if the thrust piece is form-stable under the necessary sealing forces and consists of a harder material than that of the tube. Secure and reproducible sealing conditions can be achieved in this way, wherein the active sealing forces can be led efficiently by the spring, which preferably is arranged as a perforated plate, over the thrust piece either onto another clamping element, for example a clamp ring corresponding to a tube fitting arranged as a ferrule or onto a ring flange of the tube arranged on the end of the tube.
It is particularly favorable if the thrust piece is arranged symmetrically to a middle section such that the force application point can be arranged opposite and parallel to the longitudinal axis of the tube. By doing so, the thrust piece can be prevented from tipping over and optimal centering conditions can be obtained.
The thrust piece is best arranged rotationally symmetrical and with side parts that taper radially outward. This allows for a tube connection that is particularly easy and cost-effective. The aforementioned measures allow the force application points on the thrust piece to be particularly close to the outer circumference of the tube, i.e. within the smallest possible distance to the longitudinal axis of the tube. By doing so the frictional forces and tightening momentum can be minimized and favorable sealing conditions are attainable such that co-rotation of the thrust piece can be prevented.
Such advantage is attained particularly if the thrust piece is arranged between the perforated plate and the base element and the perforated plate is arranged between the fastening element and the thrust piece.
It is particularly favorable if the thrust piece and an opposing ring flange of the tube protruding from the longitudinal axis of the tube are coordinated to each other such that an extending ring gap is formed between the thrust piece and the ring flange when the thrust piece is put on the tube and in the non-braced state, in which the thrust piece fits closely on the ring flange. In this way, when the connection elements are braced in the direction of the axis with relatively low tightening momentum, increased surface pressure can be exerted on the flange base, and the tube connections, which preferably are provided with fastening elements in the form of clamp screws, can be placed at the lowest possible distance from each other.
If the ring flange, which is connected to the tube forming one piece, consists of a polymer, preferably of a fluoropolymer, in particular PTFE or FEP, the ring flange may plastically deform under the active sealing forces. This means that the active surface can increase to the thrust piece until the respective force supply is compensated. It is therefore particularly sensible if an angle of 5 to 25 degrees, in particular an angle of approximately 10 degrees, is formed between the side parts of the ring flange and opposed side parts of the unbraced thrust piece adjacent thereto. This allows for steady surface pressure between the thrust piece and the ring flange of the tube to be achieved, both with low sealing forces or tightening momentum as well as with large sealing forces or tightening momentum respectively. This signifies a comparatively large tolerance for permissible axis bracing or permissible tightening momentum respectively. When a perforated plate is used as a spring, which preferably is arranged between the thrust piece and the active surfaces of the fastening element, it is elastically deformed over a set spring excursion and consequently can compensate a plastic deformation of the tube or ring flange of the tube respectively over its operational lifetime within the limits of the predetermined spring excursion.
According to another particularly favorable embodiment of the invention, preferable force application conditions on the one hand and favorable sealing conditions on the other hand can be achieved if the thrust piece formed symmetrically to a middle section is arranged with rotationally symmetrical side parts that taper radially outward and the perforated plate is also formed symmetrically to a middle section, preferably with parallel ring surfaces leading away from each other, and if the fastening element is formed with the ring surfaces arranged inclined toward the direction of the ring flange of the tube. When the fastening element is braced relative to the base element, the perforated plate is elastically deformed between the active surfaces of the fastening element and the active surfaces of thrust piece can thereby compensate axial defects between the braced elements, which requires comparatively low bracing force. Moreover, centering on the tube of the thrust piece or downstream fastening elements respectively can be achieved through this configuration and contingent upon elastic deformation of the perforated plate such that favorable sealing conditions are attained and unfavorable dead spaces are prevented.
The invention also concerns a use of a screw connection, which has one or several of the aforementioned characteristics individually or in combination with each other for the sealed connection of flow ducts in liquid separation technology, in particular in analytical or preparative HPLC.
In this type of use it is advisable for the tube to have an outside diameter of less than or equal to 6.5 mm, in particular less than or equal to 2 mm and an inside diameter of less than or equal to 5.5 mm, in particular 0.25 to 1.0 mm.
It is furthermore advisable if the tube has a through-flow channel that enables flow rates up to 6,000 ml/minutes, in particular 1 ml per minute to 100 ml per minute.
The aforementioned characteristics contribute both individually as well as in combination with each other to a tube connection that is both easily manageable and operable with low energy expenditure, and that guarantees a sealed flow connection between the tube and a connected flow duct part over long periods of time.